As a shift control technique of the automatic transmission which is inserted into the motor power transmission system in this way, there has been known a technique of shift-controlling the automatic transmission in accordance with a required motor torque of an electric motor, as described in Patent Literature 1.
The shift control technique as typified by the one described in the patent literature 1 determines a target deceleration to be achieved by a regenerative braking (a negative required motor torque) of the electric motor on the basis of a braking operation at the braking time. Next the shift control technique determines a required shift stage capable of achieving the target deceleration from the current transmission output rotational speed on the basis of a relation map between a shift stage of the automatic transmission which can achieve the target deceleration, and the transmission output rotational speed. And then, the shift control technique changes speed of the automatic transmission to the required shift stage.
In other words, in the case that the target deceleration is large, the automatic transmission is shifted down to the lower shift stage than the current shift stage.
Further, the shift control technique, at the accelerating time, determines a target driving force to be achieved by a driving torque (a positive required motor torque) of an electric motor on the basis of an accelerating operation, and changes speed of the automatic transmission to a lower shift stage which can achieve the target driving force.
In other words, the automatic transmission is shifted down to the lower shift stage than the current shift stage, even in the case that the target driving force is increased by the accelerating operation.